Ion implantation is a technique for introducing conductivity-altering impurities into semiconductor workpieces. During ion implantation, a desired impurity material is ionized in an ion source chamber, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is focused and directed at the surface of a workpiece positioned in a process chamber. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the material to form a region of desired conductivity.
During the ion implantation process, the ionization of impurity materials and the interaction of an ion beam with a workpiece can generate undesirable gases that must be evacuated from an ion source chamber and a process chamber, respectively, in order to prevent contamination of the chambers and the workpiece. To that end, semiconductor process pumps that are capable of rapidly evacuating such gases are commonly installed at various locations throughout ion implanters. For example, turbomolecular pumps (commonly referred to as “turbopumps”) are often installed on or adjacent to ion source chambers of ion implanters for evacuating gases (e.g., boron, arsenic, argon, etc.) that are produced during ionization of impurity materials. Similarly, cryogenic pumps (often referred to as “cryopumps”) are typically installed on process chambers of ion implanters for evacuating gases (e.g., water vapor, hydrogen, etc.) produced when an ion beam strikes a workpiece.
Semiconductor process pumps are typically expensive and can have a significant impact on the overall cost of an ion implanter. It is therefore generally desirable to reduce the total number of such pumps required. This can be done by maximizing pump speeds (i.e., the rate at which gases are evacuated by a pump) so that a fewer total number of pumps must be implemented in a particular application to achieve sufficiently rapid evacuation of gases.
Pump speed is greatly affected by the physical distance between the working surfaces of a pump (e.g., rotors and stators of turbopumps and low-temperature surfaces of cryopumps) and gases that are to be evacuated. Thus, since semiconductor process pumps are generally mounted on the exterior surfaces of ion source chamber walls and process chamber walls, the thicknesses of such walls can be a significant confounding factor with regard to pumping efficiency. For example, at pressures below 1×10−2 pascals, chamber walls having thicknesses of 1-2 inches can reduce pump efficiency by 25 percent or more of vacuum pump with a 250 mm inlet diameter. Furthermore, semiconductor process pumps typically include mounting flanges which facilitate attachment to chamber walls. Such flanges further separate the working surfaces of pumps from the interiors of ion source and process chambers and thus further reduce pump efficiency. Still further, in the case of cryopumps, a gate valve must typically be installed intermediate the inlet of a pump and a process chamber wall so that the pump can be closed off from the interior of the process chamber to facilitate periodic regeneration of the working surfaces of the pump. Such gate valves further separate the working surfaces of cryopumps from the interiors of process chambers and may further reduce pump efficiencies by as much as 30 percent.
Due to the above-described losses, it is often necessary to use multiple turbopumps and multiple cryopumps in an ion implanter in order to achieve necessary rates of gas evacuation, despite the fact that a single turbopump and/or a single cryopump may be capable of achieving the necessary evacuation rates if the full, or near full, efficiencies of such pumps could be realized during operation. Thus, there is a need for an improved arrangement for turbopumps and cryopumps that can reduce the total number of such pumps required for an ion implantation system.